A biometric authentication technology determining whether an individual is to be authenticated using biometric information such as a fingerprint or a palm print has been developed recently. The biometric authentication technology is widely used from a large scale system having a large number of registered users such as an entrance-exit management system, a system for controlling boarders, and a system using national identification system to a small scale device used by an individual such as a computer or a portable terminal.
For example, when any one of fingers is used as the biometric information, the biometric authentication apparatus acquires a biometric image representing the fingerprint as an input biometric image. The biometric authentication apparatus then matches the input biometric information which corresponds to a fingerprint of a user represented in the input biometric image with registered biometric information which corresponds to a fingerprint represented in the biometric image of a registered user who has been registered in advance. When it is determined that the input biometric information coincides with the registered biometric information based on a result of the matching process, the biometric authentication apparatus authenticates the user as a registered user who has a valid authority. Also, the biometric authentication apparatus allows the authenticated user to use an apparatus into which the biometric authentication apparatus is inserted or another apparatus connected to the biometric authentication apparatus.
When a sensor reads the biometric information, the quality of the biometric information captured on the biometric image may be degraded due to the surface state of the portion of a living body of a user including the biometric information of the user or due to the movement of the portion at the time of reading the biometric information. For example, when the user presses his or her finger excessively against the sensor, the ridge line may be squashed and the difference in brightness between a pixel corresponding to a ridge line and a pixel corresponding to valley line on the biometric image becomes smaller and thus, the fingerprint becomes unclear. Further, when the surface of the finger is wet or dry, the fingerprint captured on the biometric image may become unclear. As described above, when the biometric image having unclear biometric information is captured is used for a biometric authentication, the biometric authentication apparatus cannot accurately extract the feature amount representing the features of the biometric information. As a result, the authentication accuracy is likely to be degraded.
Therefore, a technique of correcting biometric information captured on a biometric image has been proposed. See, for example, Japanese Laid-Open Patent Publication No. 2006-72553, Japanese Laid-Open Patent Publication No. 2007-202912, Japanese Laid-Open Patent Publication No. 2011-150483 and Japanese Laid-Open Patent Publication No. H09-62836.
Japanese Laid-Open Patent Publication No. 2006-72553 discloses a technique in which input biometric information data is output as it is when correction of the biometric information data is not necessary. However, a predetermined correction process is performed on the input biometric information data to output corrected biometric information data subjected to the correction process when a correction of biometric information data is necessary. Further, in the technique, the quality of the biometric information is evaluated according to a normal evaluation criteria when the biometric information data has not been subjected to the correction process. However, the evaluation criteria is raised to evaluate the quality of the corrected biometric information data when the biometric information data has been subjected to the correction process. Also, when the quality of the biometric information data satisfies the evaluation criteria, a unique feature data extracted from the biometric information data is registered.
Japanese Laid-Open Patent Publication No. 2007-202912 discloses a technique in which the frequency component of which an absolute value of amplitude greater than or equal to a predetermined threshold value is selected, and when the selected frequency component satisfies the quality of a pixel adequate for a matching process, the pixel is reconfigured based on the selected frequency component.
Japanese Laid-Open Patent Publication No. 2011-150483 has proposed a technique in which a gradient image including a fingerprint or a palm print is emphasized based on a ridge line direction distribution data representing the direction distribution of a shape of the ridge line of a fingerprint or a palm print. Further, Japanese Laid-Open Patent Publication No. H09-62836 discloses an image processing method in which a suppressed edge strengthening process or no edge strengthening is performed for an edge region other than the edge region having a degree of edge large than a predetermined threshold value.